Synchronous electric motors such as brushless DC motors and permanent-magnetic synchronous motors (“PMSM”) or (“PMM”) are commonly used for high precision applications where careful speed and/or position control are required. Pulse width modulation (PWM) is a common approach used to control the power supplied to synchronous motors for purposes of controlling the speed and torque of the motor.
Generally, the average value of voltage (and current) fed to the inertial load of a synchronous motor can be controlled through PWM by turning the switch between the power supply and load on and off at a fast rate. Typically, PWM switching frequency has to be much higher than what would affect the load. High frequency PWM power control systems can be easily realizable with semiconductor switches because almost no power is dissipated by the switch in either the “on” or “off” state. However, during the transitions between “on” and “off” states, both voltage and current are nonzero and thus power is dissipated in the switches. By quickly changing the state between fully on and fully off (typically less than 100 nanoseconds), the power dissipation in the switches can be quite low compared to the power being delivered to the load. However, when this low power dissipation of a single switching instance is multiplied by the PWM frequency, the power dissipation can become problematic.
Modern semiconductor switches or transistors such as a MOSFET or insulated-gate bipolar transistors (IGBTs) are well suited components for high-efficiency controllers. Generally, in such applications high-side switch drivers use something called a “bootstrap” technique to produce a floating voltage to switch the gate of a semiconductor switch such as a MOSFET. This common technique can be cost effective, however, it's very limited to on-time because the bootstrap capacitor discharges rapidly. Therefore, a PWM signal must be used to turn the MOSFET on and off thousands of times a second to recharge the bootstrap capacitor. The downfall to this is that MOSFETs dissipate the most energy as heat in the on-off or off-on transition.
This wasted energy is commonly referred to as switching loss. In general, if heat dissipation isn't properly maintained, the switching losses can cascade to the point of complete device failure. Although heat sinks can be commonly used to remedy this problem, PWM control can still be susceptible to higher switching losses which results in lower controller efficiency.
Another common circuit used to produce a voltage higher than the bus voltage to drive the gate of a high-side switch is a charge pump. The basic charge pump is a circuit that switches back and forth between two capacitors, charging one while using the other, to maintain a certain voltage. Due to component limitations with regard to low power capability and limited output-voltage options, as well as cost concerns, the charge pump is commonly only useful in low voltage applications.
Accordingly, there is a need in the art for a high efficiency motor control system that can be reliable and efficient across a wide range of motor loads and speeds. There is additionally a need for such a control system to be cost effective, flexible and robust by being able to minimize heat dissipation and switching loss which can commonly contribute to device failure.